Apparatus for conducting simulated countercurrent adsorptive separation of a multi-component feed stream

ABSTRACT

A process for separating a product from a multicomponent feedstream to an adsorption apparatus or system is described. The apparatus or system may comprise a moving-bed or a simulated moving-bed adsorption means. The product comprises at least one organic compound, such as an aryl compound with alkyl substitutes. In embodiments the conduits used to supply the feedstream to the apparatus or system are flushed with media of multiple grades. The improvement is more efficient use of the desorbent. In embodiments the process achieves improvements in one or more of efficiency of adsorption separation, capacity of adsorption apparatus systems, and purity of product attainable by adsorption process.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.13/070,009, now allowed, which claims the benefit of U.S. ProvisionalApplication No. 61/319,080, filed Mar. 30, 2010, and U.S. applicationSer. No. 12/774,319, filed May 5, 2010, the disclosures of which areincorporated herein by reference in their entireties.

FIELD OF THE INVENTION

The invention relates to a process for separating one or more of thecomponents from two or more multicomponent fluid mixtures, and moreparticularly to a process for separating organic compounds from such afluid mixture by means of adsorption apparatus, such as moving-bed orsimulated moving-bed adsorption apparatus, and a system comprising suchapparatus.

BACKGROUND OF THE INVENTION

Various means are currently available to separate the components of amulticomponent fluid mixture. If the densities of the components differsufficiently, the effects of gravity over time may be adequate toseparate the components. Depending on the quantities of the componentsinvolved, a centrifuge may be used to more rapidly separate componentswith different densities. Alternatively, distillation may be used toseparate components with different boiling points.

Some fluid mixtures comprise components which have similar boilingpoints, and in such cases, separation by distillation may be a difficultand an inefficient means to separate these components. Too manycontaminants, e. g., unwanted components, also may evaporate along with(or fail to evaporate from) the desired component(s), or the separationmay require high energy expenditures due to the recycling through thedistillation process that may be necessary to attain a desired degree ofseparation or purity.

In view of these and other deficiencies of these aforementionedprocesses, adsorption often has been preferred as a process forseparating the components from a multicomponent fluid mixture to obtainrelatively pure products.

The efficiency of an adsorption process may be partially dependent uponthe amount of the surface area of the adsorbent solids which isavailable for contact with a fluid mixture. The surface area availablemay be more than just the superficial, external surface of the solids.Suitable solids also may have internal spaces. Such internal spaces maycomprise pores, channels, or holes in the surface of the solids and mayrun throughout the solids, much as in sponges. Thus, the fluid contactsnot only the superficial surface, but penetrates into the solids. Sievechambers increase the contact surface between the fluid and the solidsin an adsorption process by concentrating them in a confined space. Suchstructures often are described as molecular sieves, and the volumetricamount of components that may be adsorbed by a molecular sieve is termedthe molecular sieve capacity.

In an adsorption process, separation of the fluid components may beaccomplished because the absorbent solid material may have a physicalattraction for one or more of the components of the mixture inpreference to other components of the mixture. Although all of thecomponents of a mixture may be attracted in varying degrees to thematerial, there is a preference engineered into the process, such thatpredominantly the desired component(s) may be attracted and remain withthe material in preference over all others. Therefore, even if lesspreferred components of a mixture initially come into contact with aportion of the material, because of the stronger attraction of thematerial for the desired component(s) of the mixture, the less preferredcomponent(s) may be displaced from the material by the desired, and morestrongly preferred, component(s). Although the fluid mixture entering asieve chamber might be composed of multiple components, the fluidmixture initially leaving the vessel would be composed largely of thecomponents which had been less preferentially adsorbed into thematerial.

In adsorption processes using adsorbent solids, separation occurs for aperiod of time, but eventually all the available surface sites on and inthe solids are taken up by the desired component(s) or are blocked byconcentrations of unwanted components. At that point, little significantadditional adsorption of component(s) from the mixture is likely tooccur, and the fluid mixture which might be withdrawn from the chambermay be insignificantly changed by further exposure to the solids. Theadsorption step of the process is thus ended, and the component(s) whichhave been adsorbed by the solids can then be removed from the solids, soas to effect separation and permit reuse of the solids.

A suitable adsorption apparatus or system might first permit adsorptionof a product comprising the desired component(s) by the solids and latertreat the solids to cause them to release the product and permitrecovery of this product. Such an adsorption apparatus or system mightcomprise a “moving-bed” which permits movement of a tray or bed of thesolids through a chamber, such that at different locations, the solid issubjected to different steps of an adsorption process, e.g., adsorption,purification, and desorption. These steps will be understood moreclearly by the description below. Nevertheless, moving the solidsthrough an adsorption apparatus may be difficult and involve complexmachinery to move trays or beds. It also may result in loss of thesolids by attrition. To avoid these problems, some adsorption apparatusand systems have been designed to “simulate” moving the tray(s) orbed(s) to the locations, e.g., zones, of different steps of anadsorption process. Simulation of the movement of the tray(s) or bed(s)may be accomplished by use of a system of conduits which permitsdirecting and redirecting the streams of fluids into the chamber atdifferent zones at different times. As these stream changes occur, thesolids are employed in different steps in an adsorption process asthough the solids were moving through the chamber.

The different zones within an adsorption apparatus or system are definedby the particular step of the adsorption process performed within eachzone, e.g., (1) an adsorption step in the adsorption zone; (2) apurification step in the purification zone; (3) a desorption step in thedesorption zone. A more detailed explanation of the zones of theadsorption process follows.

Adsorption Zone: when a multicomponent fluid feedstream, such as afeedstream comprising the C8 aromatics orthoxylene (OX), metaxylene(MX), paraxylene (PX), and ethylbenzene (EB), is fed into the adsorptionapparatus or system, the portion of the apparatus or system into whichthe feedstream is being fed is termed an “adsorption zone.” In theadsorption zone, the fluid comes into contact with the adsorbentmaterial, and the desired component(s) are adsorbed by the adsorbentmaterial. As noted above, other components may also be adsorbed, butpreferably to a lesser extent. This preferential adsorption may beachieved by the selection of an adsorbent material, e.g., adsorbentsolids, which have a preference for adsorbing the desired component(s)from the multicomponent feedstream. Although only the desiredcomponent(s) may have been adsorbed by the solids, other lesspreferentially adsorbed components of the fluid mixture may still remainin void spaces between the solids and possibly, in the pores, channels,or holes within the solids. These unwanted components preferably areremoved from the solids before the desired component(s) are recoveredfrom the solids, so that they are not recovered along with the product.

Purification Zone: after adsorption, the next step is to purify thefluid and adsorbent material in the chamber. In this step, the tray(s)or bed(s) may be moved or flow within the conduits may be changed, sothat the multicomponent feedstream may no longer be fed into theadsorption zone. Although the tray(s) or bed(s) have not physicallymoved, the material may now be described as being in a “purificationzone” because a fluid stream, e.g., a purification stream, is fed intothe adsorbent material to flush the unwanted components from theadsorbent material, e.g., from within and from the interstitial areasbetween the solids. Thus, a fluid comprising unwanted components, e.g.,raffinate, is flushed from the purification zone by substituting a fluidcomprising the desired component(s) or other component(s) deemed to bemore acceptable for the unwanted components. The unwanted components maybe withdrawn in a raffinate stream. Because an objective of theadsorption process may be to separate the product comprising the desiredcomponent(s) from other components which may have nearly the sameboiling point or density as the desired component(s), purification maydisplace unwanted components and substitute another fluid which can bemore readily separated by other means, e.g., distilled.

Desorption Zone: after the solids have been subjected to thepurification stream, the stream in the conduit(s) may again be changedto introduce a desorbent stream into the chamber to release the product.The desorbent stream contains desorbent which is more preferentiallyadsorbed by the solids than the product comprising the desiredcomponent(s). The desorbent chosen will depend in part upon the desiredcomponent(s), the adsorbent materials, and the ease with which thedesorbent can be separated from the product. Once the desorbent streamhas been introduced to the chamber, the product may be withdrawn fromthe chamber.

Each and every step and zone might be present somewhere in an adsorptionapparatus or system if simultaneous operations are conducted.Nevertheless, the steps may be performed successively or staggered overtime. Further, in some adsorption processes, the unwanted components maybe adsorbed, and the product comprising the desired component(s) allowedto pass through the adsorption apparatus or system. Therefore, the termsraffinate and extract are relative and may depend upon the particularnature of the components being separated, the preference of the solids,and the nature of the apparatus or system. Although in embodiments thepresent invention will be discussed primarily in terms of apparatus andsystems in which the product is adsorbed by the solids, the invention isnot limited to such configurations.

An apparatus suitable for accomplishing the adsorption process of thisinvention is a simulated moving-bed adsorption apparatus. A commercialembodiment of a simulated moving-bed adsorption apparatus is used in thewell-known Parex™ Process, which is used to separate C8 aromatic isomersand provide a more highly pure paraxylene (PX) from a less highly puremixture. See by way of example U.S. Pat. Nos. 3,201,491; 3,761,533; and4,029,717.

Typically, such an adsorption apparatus is contained in a verticalchamber packed with adsorbent solids, possibly in trays or beds stackedwithin the chamber. More than one type of solid also might be used. Thechamber also may have the capability to perform each of theabove-described steps simultaneously within different locations, e.g.,zones, in the chamber. Thus, the composition of the fluid in the chambermay vary between zones although there may be no structures completelyseparating these zones. This may be achieved by the use of a seriallyand circularly interconnected matrix of fluid communication conduitsincluding associated valves, pumps, and so forth, which permit streamsto be directed and redirected into different zones of the chamber and tochange the direction of these streams through the solids within thedifferent zones of the chamber. The different zones within the chambermay have constantly shifting boundaries as the process is performed.

The cyclic advancement of the streams through the solids in a simulatedmoving-bed adsorption apparatus may be accomplished by utilizing amanifold arrangement to cause the fluid to flow in a counter currentmanner with respect to the solids. The valves in the manifold may beoperated in a sequential manner to effect the shifting of the streams inthe same direction as overall fluid flow throughout the adsorbentsolids. In this regard see U.S. Pat. No. 3,706,812. Another means forproducing a countercurrent flow in the solid adsorbent is a rotatingdisc valve by which the streams, e.g., feed, extract, desorbent,raffinate, and line flush, are advanced cyclically in the same directionthrough the adsorbent solids. Both U.S. Pat. Nos. 3,040,777 and3,422,848 disclose suitable rotary valves. Both suitable manifoldarrangements and disc valves are known in the art. More recently, asystem has been described using dual rotary valves. See U.S. applicationSer. No. 12/604,836.

Normally there are at least four streams (feed, desorbent, extract, andraffinate) employed in the procedure. The location at which the feed anddesorbent streams enter the chamber and the extract and raffinatestreams leave the chamber are simultaneously shifted in the samedirection at set intervals. Each shift in location of these transferpoints delivers or removes liquid from a different bed within thechamber. In many instances, one zone may contain a larger quantity ofadsorbent material than other zones. Moreover, zones other than thosediscussed above may also be present. For example, in someconfigurations, a buffer zone between the adsorption zone and thedesorption zone may be present and contain a small amount of adsorbentmaterial relative to the zones surrounding it. Further, if a desorbentis used that can easily desorb extract from the adsorbent material, onlya small amount of the material need be present in the desorption zone incomparison to the other zones. In addition, the adsorbent need not belocated in a single chamber, but may be located in multiple chambers ora series of chambers.

Introducing and withdrawing fluids to the beds may comprise a pluralityof fluid communication conduits, and the same fluid communicationconduit may be used in a first instance to input a feedstream into theapparatus or system and later to withdraw an extract stream. This canresult in reduced product purity due to contamination of the withdrawnproduct. Fluid communication conduits may contain unwanted components,such as residue remaining in the conduit from earlier additions orwithdrawals of streams. This problem may be overcome by employingseparate conduits for each stream or by removing such residue from theconduits by flushing them with a medium which would not effect productpurity as adversely as would an unwanted component remaining in thefluid communication conduit. A preferred flushing medium has been theproduct or the desorbent, which might be more readily separateddownstream of the chamber than would the residue. See U.S. Pat. No.4,031,156. Nevertheless, flushing conduits with the product reduces theoutput of the adsorption process.

A standard Parex™ unit for separating paraxylene (PX) from the other C8aromatic isomers, metaxylene (MX), orthoxylene (OX), and ethylbenzene(EB), has a single feed to a single rotary valve or parallel rotaryvalves. The rotary valve directs the feed to a bed line, which (viewedschematically, such as in the attendant drawings described herein) issomewhere between the extract (which may comprise, by way of example,99.7% PX and desorbent) and the raffinate (PX-depleted xylenes anddesorbent) withdrawal points. Since the process is a simulated movingbed process, the bed lines are shared with all of the feed and productstreams, and therefore the bed lines must be flushed between the feedinjection point and the extract withdrawal point in order to preventcontamination of the product. A standard unit has a primary flush whichremoves the majority of contaminants and a secondary flush which removestrace impurities just before the extract point.

The standard commercial simulated moving bed has only a single feedinlet, various streams of different compositions are typically blendedtogether and fed to a single point in the Parex process. However, asindicated in U.S. Pat. No. 5,750,820 (see also U.S. Pat. No. 7,396,973),it is better to segregate feeds which are of substantially differentcomposition, such as concentrated paraxylene from a selective toluenedisproportionation unit (generally 85-90% paraxylene) and equilibriumxylenes (generally about 23% paraxylene) from a powerformer,isomerization unit or transalkylation unit. This can be done by usingthe primary line flush as a second feed point for the paraxyleneconcentrate and using the secondary flush as the sole flushing stream.Having only a single flush does result in a slight compromise in theseparation process, but the compromise typically is far outweighed bythe benefit of optimizing the feed location of the paraxyleneconcentrate as far as net purity in the final product.

There is a problem with the above configuration in that the standardParex unit has the secondary flush located close to the extractwithdrawal point in order to minimize contaminants that are withdrawnwith the extract. However, when the secondary flush is very close to theextract withdrawal point and concentrated paraxylene (having associatedimpurities) is being flushed from the bed line, the configuration willbe too close to the extract withdrawal point and the highest separationof the feed will not be realized.

This problem was recently recognized and solved by some of the presentinventors. The solution is that the feed locations of both theconcentrated paraxylene in the primary flush and also the location ofthe secondary flush be modified to realize the full benefit of the feedconfiguration in U.S. Pat. No. 5,750,820. By moving the secondary flushfurther away from the extract, the material flushed from the bed linewill be injected at a more efficient location. See U.S. application Ser.No. 12/774,319. The problem and solution are noted in the description ofFIG. 1, herein below.

All of these processes are still very energy-intensive due, at least inpart, to the intensive use of materials such as the desorbent, which istypically reused after purification downstream of the bed systemsdescribed above. It would be very beneficial if all of the systemsdescribed could be modified simply so that energy requirements could bereduced. The present inventors have realized that yet furtherimprovements can be achieved by directly using the first flush output asthe secondary flush input. This provides, in embodiments, the ability tocompletely eliminate purification requirements of the prior art, withits attendant distillation apparatus and pumping equipment, simplifyingthe system, decreasing costs, and improving the results.

SUMMARY OF THE INVENTION

The invention is directed to a process for separating a product from atleast one multicomponent feed by simulated countercurrent adsorptiveseparation, and to the apparatus or system for accomplishing the same,said process comprising at least two flushing steps to improve productpurity, wherein the improvement comprises using the first flush outputas the second flush input. In preferred embodiments, there is no step ofpurification, such as by distillation, the primary flush out materialbefore it is used as secondary flush in.

In embodiments, the process comprises feeding at least two differentfeeds, the feeds characterized by having different concentrations of atleast one product, preferably a C8 species selected from one or moreisomers of xylene. It will be recognized by one of skill in the art thata continuous simulated countercurrent adsorptive separation system canhave many desired end products, such as pharmaceuticals, fragrances,sugars, and the like.

In embodiments, the feed location of both the primary flush and/or alsothe secondary flush are altered as compared with the prior art torealize the fullest benefit of the present invention.

In embodiments the conduits used to supply the feedstream to theapparatus or system are flushed with media of multiple grades.

In embodiments the process achieves improvements in one or more ofefficiency of adsorption separation, capacity of adsorption apparatussystems, decrease in energy requirements (particularly in the aspects ofpumping and distillation requirements, and purity of product attainableby adsorption process.

In an embodiment, the process comprises the steps of: (a) introducing afirst multicomponent feed, comprising at least one desired product,through at least one fluid communication conduit into a simulatedmoving-bed adsorption apparatus comprising at least one rotary valve andplural sieve chambers; (b) flushing the at least one conduit in step (a)with at least one initial flushing medium (which preferably comprisesthe at least one desired product in step (a) in an initialconcentration), whereby residue of said first multicomponent feed isflushed from the at least one conduit in step (a) into the apparatus bythe at least one initial flushing medium, so as to produce a primaryflush out comprising said at least one initial flushing medium and saidresidue of said first multicomponent feed; (c) after step (b), flushingsaid at least one fluid communication conduit with a second andpreferably final flushing medium, characterized in that said secondflushing medium comprises said primary flush out. In preferredembodiments there is no step of distillation of said primary flush outprior to use as said second flushing medium.

In preferred embodiments, the quantity of the initial medium may not beless than that sufficient to flush the feedstream residue from theconduit.

In embodiments, the apparatus comprises plural sieve chambers containingone or more adsorbent material selected from the group consisting ofcharcoal, ion-exchange resins, silica gel, activated carbon, zeoliticmaterial, and the like, and the quantity of the initial medium may besufficient to fill the apparatus to the sieve chamber capacity.

In embodiments, the process includes additional steps including one ormore of flushing one or more conduits with a sufficient quantity of afinal (or third) flushing medium comprising the at least one desiredcomponent in a final concentration, such that the final concentration isgreater than the initial concentration and greater than the secondconcentration, and such that initial medium residue from the at leastone initial medium is flushed from the conduit into the system by thefinal medium; withdrawing a raffinate stream from the system;introducing a desorbent stream to the system; withdrawing a combinationcomprised of the product and the desorbent from the system; and removingthe product from the combination.

In yet another embodiment, the initial concentration of the at least oneinitial medium is continuously increased during the flushing of the atleast one conduit until the initial concentration equals the finalconcentration. Preferably, this may be accomplished by adding theproduct to the at least one initial medium in gradually increasingamounts and decreasing proportionately flow from the source of the atleast one initial medium; the improvement comprises using at least aportion of the first flush output as at least part of said secondflushing medium.

As is well-known per se in the commercial Parex™ unit, moving thelocations of liquid input and output is accomplished by a fluiddirecting device known generally as a rotary valve which works inconjunction with distributors located between the adsorbent sub-beds.The rotary valve accomplishes moving the input and output locationsthrough first directing the liquid introduction or withdrawal lines tospecific distributors located between the adsorbent sub-beds. After aspecified time period, called the step time, the rotary valve advancesone index and redirects the liquid inputs and outputs to thedistributors immediately adjacent and downstream of the previously useddistributors. Each advancement of the rotary valve to a new valveposition is generally called a valve step, and the completion of all thevalve steps is called a valve cycle. The step time is uniform for eachvalve step in a valve cycle, and is generally from about 60 to about 90seconds (although it can be longer or shorter). A typical processcontains 24 adsorbent sub-beds, 24 distributors located between the 24adsorbent sub-beds, two liquid input lines, two liquid output lines, andassociated flush lines. In an embodiment of the present invention, animprovement is provided whereby the rotary valve is replumbed so thatthe input of the secondary flush is a least one and preferably two ormore valve steps downstream of where it is, heretofore, ordinarilyinputted. This is more fully illustrated by the description of FIG. 1below. This also means that the secondary flush is added closer insequence to the input of the primary flush, such as within three cyclesteps.

It is an object of the invention to decrease pumping and distillationrequirements in the purification of material utilizing a simulatedmoving bed adsorptive separation system.

It is another object of the invention, in one or more embodiments, toincrease the efficiency of adsorption apparatus or systems, wherebycontaminants, such as feedstream residue, may be removed from fluidcommunication conduits by flushing them from the conduits into theapparatus or system with flushing media containing concentrations of thedesired component(s) of the product which are higher than that of thefeedstream. It is an advantage of such embodiments that if the productis extracted through the same conduits that carried the feedstream, suchas in a simulated moving-bed adsorption apparatus, extract will not becontaminated, or will have lower contamination, with feedstream residue.

It is an additional object of this invention to increase the capacity ofan adsorption apparatus or system. It is an advantage of this processthat excess capacity of the apparatus or system may be more fullyutilized by purifying the solids with flushing media and flushingconduits with media containing the desired component(s). It is a featureof such embodiments that fluid communication conduits may be flushedwith media containing concentrations of a desired component orcomponents higher than that of the feedstream, which may be drawn from asource other than the apparatus.

It is yet another object of this invention, in embodiments, to increasethe purity of the product obtained from an adsorption apparatus orsystem. It is a feature of embodiments of this process that contaminantsmay be removed from conduits and from pores, channels, and holes inadsorbent solids, and conduits may be charged with the product. It is anadvantage such embodiments that the product may be recycled through theapparatus or system, and excess apparatus or system capacity may be usedto further separate other unwanted components of the feedstreamremaining in the product.

It is yet still further an object of this invention to eliminate orreduce the circulation requirements of the desorbent, includingdownstream distillation and separation of the adsorbent from the varioussolutes contained therein, the solutes including the desired component(such as, in the case of xylenes, one particular isomer, which isgenerally paraxylene).

These and other objects, features, and advantages will become apparentas reference is made to the following detailed description, preferredembodiments, examples, and appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings, like reference numerals are used to denotelike parts throughout the several views.

FIG. 1 is a schematic illustrating a comparison of variousconfigurations of simulated moving bed adsorptive separation systems.

FIG. 2 is a schematic illustrating one embodiment of a separation systemaccording to the present invention.

DETAILED DESCRIPTION

According to the invention, there is provided a process for separating aproduct from at least two multicomponent feeds to an adsorptionapparatus or system. In an embodiment there is a simulated moving bedadsorptive separation system plumbed so that the first or primary flushoutput is connected to the secondary flush input directly, whereby theprimary flush output, comprising desorbent and having a higherconcentration of the desired compound as compared to the flush materialprior to being used as primary flush, is then utilized as secondaryflush input, resulting, in more preferred embodiments, in the secondaryflush output comprising desorbent and having even higher concentrationof the desired compound than in the primary flush input.

The apparatus or system may comprise a moving-bed or a simulatedmoving-bed adsorption means, and in embodiments provides a productcomprising at least one organic compound, such as an aryl compound withalkyl substitutes, for instance (and in a preferred embodiment),paraxylene (PX). In embodiments the conduits used to supply thefeedstream to the apparatus or system are flushed with media of multiplegrades. In embodiments the process achieves improvements in one or moreof efficiency of adsorption separation, capacity of adsorption apparatussystems, and purity of product attainable by adsorption process, whileallowing for elimination or by-pass of distillation apparatus and/orpumping mechanisms.

In embodiments the feed location of both the concentrated paraxylene inthe primary flush and also the location of the secondary flush arelocated to realize the full benefit of feed locations. In embodiments,by moving the secondary flush further away from the extract, thematerial flushed from the bed line will be injected into a moreadvantageous point in the profile. This allows for additional capacityor decreased use of energy associated with a decrease of desorbentrecirculation will be realized.

As described more fully in U.S. application Ser. No. 12/774,319, asystem employing a simulated countercurrent flow process such asdescribed in U.S. Pat. Nos. 3,201,491; 3,761,533; and 4,029,717, isshown in FIG. 1, along with several modifications. The diagram in FIG. 1will be understood by those of skill in the art to depict a simulatedmoving bed process. Desorbent is introduced through conduit 100, flushleaves the apparatus through flush out conduit 101, extract (containingthe desired product) leaves the apparatus via conduit 102, raffinateleaves the system through conduit 110, the secondary flush is addedthrough conduit 103, the primary flush is added through conduit 106, afirst multicomponent feed is added to the system through conduit 107 andoptionally a second multicomponent feed is added through lines 108 or109, as explained more fully in the following description.

Not shown in the drawing, but as would be recognized by one of skill inthe art in possession of the disclosure of U.S. application Ser. No.12/774,319, is one or more distillation towers and attendant pumps andconduits, which are utilized to purify the flush leaving theabove-described apparatus via conduit 101. It has been recognized by thepresent inventors that such downstream operations can be minimized orentirely omitted by rerouting (such as by replumbing or retrofitting)the primary flush 101 to be used as secondary flush 103. The presentinventors have realized that the primary flush 101 has the necessarycharacteristics of a useful secondary flush, thus avoiding at least partof the circulation of desorbent, with attendant savings in energy,equipment, while at the same time, in embodiments, providing an improvedproduct, e.g., purified PX. An embodiment of the invention will be downfurther below, with reference to FIG. 2.

Continuing with the description of FIG. 1, the adsorbent 112 movesupward through the sieve chamber vessel 120 containing plural bedlinesA₁ through A_(n+j). The hydrocarbon liquid feed 111 flows countercurrentto the circulating adsorbent. In operation, the adsorbent does not flow,but the various feed and product streams cycle through the bed lines,represented by lines A₁ through A_(n+j), at a rate that is differentthan the circulating hydrocarbon. This simulates the movement of the bedlines A₁ through A_(n+j). Theoretically there may be any number of bedlines, thus n>2 and n+j is the maximum number of bedlines, however froma practical standpoint the number of bed lines is limited by designconsiderations and other factors. A further discussion may be found inthe prior art too numerous to mention, but by way of example the patentsdiscussed in the background above and references cited therein. What isimportant is the relative positions of the bedlines caused by thestepping of the rotary valve, as would be understood by one of skill inthe art (such as that n and j are positive integers and that in typicalcommercial embodiments the total number of bedlines is 24, and thus n+jtypically will be 24). Certain bedlines, i.e., bedlines between A₂ andbedlines A_(n+3), A_(n+5), A_(n+6), and A_(n+10) through A_(n+j−1) arenot depicted in FIG. 1, for convenience of view.

In a conventional unit, the sieve preferentially starts adsorbing theparaxylene molecules from feed 107 in bedline A_(n+9) and flows upward.In embodiments, the feed is selected from the group consisting ofequilibrium xylenes (such as from a powerformer, isomerization unit ortransalkylation unit), which is about 21-24 wt % PX, concentrated PXfrom a selective toluene disproportionation unit (STDP unit), which isabout 85-90 wt % PX, and admixtures thereof.

The paraxylene is desorbed from the sieves in the bedlines by desorbentstream 100, the main component of which also is strongly adsorbed on thesieve(s) in bedlines A₁ through A_(n+j), but has a different boilingpoint and is easily separated from the desired product(s) downstream ofthe apparatus. In embodiments, the desorbent is paradiethylbenzene(PDEB), toluene, or a mixture thereof, or some other strongly adsorbedcompound.

The extract 102, which in the embodiment described is a mixture of thepurified paraxylene and the desorbent, is withdrawn at a point betweenthe feed 107 and the desorbent 100. The raffinate 110 consists of theparaxylene-depleted xylenes and desorbent.

Because this is a simulated moving bed process, the various feeds andproducts must share the lines between the bedlines (sieve beds) androtary valve (not shown). To prevent loss of paraxylene molecules in thehydrocarbon recycle 111 and subsequently the raffinate 110, the bedlines between the extract out 102 and desorbent in 100 are flushed out,with flush out leaving via conduit 101. The flush out can either be sentto the extract tower for recovery or recycled and used for primary flushin 106.

In addition (and more importantly), since feed 113 is routed through thetransfer lines (not shown) between the rotary valve (also not shown) andthe sieve chambers A₁ through A_(n+j) before extract 102 the transferline should be thoroughly flushed to avoid contamination of the productextract 102. Either finished paraxylene product or desorbent, e.g., PDEBor toluene, are routed through a primary flush 114 and a secondary flush104 (or alternatively 105, as discussed in more detail herein). Thesecondary flushing step through line 104 is just before the extractwithdrawal location 102 in order to flush any trace amounts ofcontaminants that may have leaked from the sieve chamber(s) back intothe bed lines.

As taught in U.S. Pat. No. 5,750,820, it is an improvement to segregateconcentrated paraxylene, such as may be obtained downstream from 102 bydistillation and then recycle, and use it as primary flush 114 in placeof location 106. This step is beneficial because it routes theconcentrated paraxylene into a more optimum place in the compositionprofile. In addition, while the concentrated paraxylene 109 is not aspure as desorbent 100 or product paraxylene (extract 102 is acombination of product para-xylene and desorbent, which can be separateddownstream such as by distillation), it does reduce the amount ofcontaminants in the bed lines A₁ through A_(n+j) and facilitate thesecondary flushing step.

However, flushing a bed line (i.e., A₁ through A_(n+j)) full ofconcentrated PX (85-90 wt %) right next to the extract creates a problemwhich was unanticipated and not even recognized by the inventors of theaforementioned improvement. Without wishing to be bound by theory, theproblem is that following the teaching of U.S. Pat. No. 5,750,820 theflush, in operation, may have about 10 vol % impurities whereas what isneeded is closer to zero, such as 0.5 vol % or less impurities.

In previously mentioned U.S. application Ser. No. 12/774,319, theinventors of that case, having discovered the aforementioned problem,proposed that in order to realize the full benefit of the movement ofthe input of the concentrated stream in line 108 from 107 to 106 vialine 109, the feed location of the secondary flush 103 must be moved toan improved place in the composition profile, e.g., further from 104,and closer to 113. One embodiment of such is depicted in FIG. 1 as line105.

Again, it should be emphasized, as would be known by one of skill in theart, that these positions are relative and that, although the actualpositions change by virtue of the movement of the rotary valve (notshown), the relative positions of the lines remains the same. Thus, itwill be understood by one of ordinary skill in the art that FIG. 1depicts a simplified simulated moving-bed apparatus with a rotary valve,wherein countercurrent “movement” of the solids in bed lines A₁ throughA_(n+j) relative to the fluid streams is simulated by the use of therotary valve, which is not shown in the figure. As the valve rotates,the zones previously discussed move through the column in a stepwisesequence due to the change in the stream flows through the valve. Inembodiments, a preferred rotary valve for performing this invention isdescribed in U.S. Pat. No. 3,205,166. In this arrangement, each fluidcommunication conduit connected to the chamber may serve a differentfunction with each step rotation of the rotary valve.

Likewise, with respect to FIG. 2, which is a schematic illustrating anembodiment of the present invention, it will be understood by one ofordinary skill in the art in possession of the present disclosure thatFIG. 2 also depicts a simplified simulated moving-bed apparatus with arotary valve (not shown for convenience of view), wherein countercurrent“movement” of the solids in bed lines B₁ through B_(n+j) relative to thefluid streams is simulated by the use of the rotary valve, which is notshown in the figure. As the valve rotates, the zones previouslydiscussed move through the column in a stepwise sequence due to thechange in the stream flows through the valve. As in FIG. 1, inembodiments, a preferred rotary valve for performing this embodiment ofthe present invention as shown in FIG. 2 is described in U.S. Pat. No.3,205,166. In this arrangement, each fluid communication conduitconnected to the chamber may serve a different function with each steprotation of the rotary valve.

Desorbent is introduced through conduit 200, flush leaves the apparatusthrough conduit 201, extract (containing the desired product) leaves theapparatus via conduit 202, and raffinate leaves the system throughconduit 210. However, unlike in FIG. 1, conduit 201 can be replumbed soas to connect to the conduits 204 and/or 205, analogous to conduit 103in FIG. 1, to provide secondary flush.

The primary flush is added through conduit 206, a first multicomponentfeed is added to the system through conduit 207 and optionally a secondmulticomponent feed is added through lines 208 or 209, as explained morefully in the following description.

The adsorbent 212 moves upward through the sieve chamber vessel 220containing plural bedlines B₁ through B_(n+j). The hydrocarbon liquidfeed 211 flows (schematically) countercurrent to the adsorbent. Inoperation, the adsorbent does not flow, but the various feed and productstreams cycle through the bed lines, represented by lines B₁ throughB_(n+j), at a rate that is different than the circulating hydrocarbon.This simulates the movement of the bed lines B₁ through B_(n+j).Theoretically there may be any number of bed lines, thus n>2 and n+j isthe maximum number of bedlines, however from a practical standpoint thenumber of bed lines is limited by design considerations and otherfactors. As mentioned above with respect to FIG. 1, a further discussionof these details may be found in the prior art too numerous to mention,but by way of example the patents discussed in the background above andreferences cited therein. Again, what is important is the relativepositions of the bedlines caused by the stepping of the rotary valve, aswould be understood by one of skill in the art (such as that n and j arepositive integers and that in typical commercial embodiments the totalnumber of bedlines, n+j, is 24). Certain bedlines, i.e., bedlinesbetween B₂ and B_(n), bedlines B_(n+3), B_(n+5), B_(n+6), and B_(n+10)through B_(n+j−1) are not depicted in FIG. 2, for convenience of view.

As in a conventional unit, the sieve preferentially starts adsorbing theparaxylene molecules from feed 207 (bedline B_(n+9)) and flows upward.In embodiments, the feed is selected from the group consisting ofequilibrium xylenes (such as from a powerformer, isomerization unit ortransalkylation unit), which is about 21-24 wt % PX, concentrated PXfrom a selective toluene disproportionation unit (STDP unit), which isabout 85-90 wt % PX, and admixtures thereof.

The paraxylene is desorbed from the sieves in the bedlines by desorbentstream 200, the main component of which also is strongly adsorbed on thesieve(s) in bedlines B₁ through B_(n+j), but has a different boilingpoint and is easily separated from the desired product(s) downstream ofthe apparatus. In embodiments, the desorbent is paradiethylbenzene(PDEB), toluene, or a mixture thereof, or some other strongly adsorbedcompound.

The extract 202, which in the embodiment described is a mixture of thepurified paraxylene and the desorbent, is withdrawn at a point betweenthe feed 207 and the desorbent 200. The raffinate exits conduit 210,which consist of the paraxylene-depleted (less strongly adsorbed)xylenes and desorbent.

Because this is a simulated moving bed process, the various feeds andproducts must share the lines between the bedlines (sieve beds) androtary valve (not shown). To prevent loss of paraxylene molecules in thehydrocarbon recycle 211 and subsequently the raffinate 210, the bedlines between the extract out 202 and desorbent in 200 are flushed outthrough conduit 201. Appropriately plumbed, which is within the skill ofthe ordinary artisan in possession of the present disclosure, the flushout can be either be sent to the extract tower for recovery or recycled,used for primary flush in 106, or, as in the embodiment of the presentinvention, used as secondary flush in through conduit 204 and/or conduit205. All options may be provided the operator, allowing maximumflexibility.

In addition, since feed 213 is routed through the transfer lines betweenthe rotary valve and the sieve chambers B₁ through B_(n+j) (none ofwhich is shown in the figure for convenience of view but per se usingconventional plumbing) before extract 202 the transfer line should bethoroughly flushed to avoid contamination of the product extract 202.Either finished paraxylene product or desorbent, e.g., PDEB or toluene,may still be routed through a primary flush 214 and a secondary flush204, if desired at predetermined times. The secondary flushing stepthrough line 204 is just before the extract withdrawal location 202 inorder to flush any trace amounts of contaminants that may have leakedfrom the sieve chamber(s) back into the bed lines.

The invention has been described above with reference to numerousembodiments and specific examples. Many variations will suggestthemselves to those skilled in this art in light of the above detaileddescription. All such obvious variations are within the full intendedscope of the appended claims.

Trade names used herein are indicated by a™ symbol or ® symbol,indicating that the names may be protected by certain trademark rights,e.g., they may be registered trademarks in various jurisdictions. Allpatents and patent applications, test procedures (such as ASTM methods,UL methods, and the like), and other documents cited herein are fullyincorporated by reference to the extent such disclosure is notinconsistent with this invention and for all jurisdictions in which suchincorporation is permitted. When numerical lower limits and numericalupper limits are listed herein, ranges from any lower limit to any upperlimit are contemplated.

What is claimed is:
 1. An apparatus adapted for a process for separatinga product from at least one multicomponent feed by simulatedcountercurrent adsorptive separation comprising plural sieve chambersfluidly connected with plural bedlines A₁ through A_(n+j), wherein n>2and n+j is the total number of bedlines, which in turn are fluidlyconnected with a matrix of fluid communication conduits to distributeplural input streams, including desorbent, a primary flush, a secondaryflush, and at least one multicomponent feed, and plural output streams,including a primary flush output and a secondary flush output, extractcomprising the product, and raffinate, from said plural sieve chambers,the improvement characterized by a fluid connection between the primaryflush output communication conduit and the secondary flush inputcommunication conduit, whereby the primary flush output may be routeddirectly for use as the secondary flush without being routed throughintervening distillation apparatus, and wherein the secondary flushinput enters a bedline (A_(n+4)) four bedlines upstream of the bedline(A_(n)) from which the extract is withdrawn.